Glaciers and ice sheets play an active role in the climate system and the global hydrological cycle. The stability of continental ice sheets must be better understood for assessments of future sea level rise and to uncover the causes of millennial-scale climate variability that characterized the last glacial period. Ice-rafted debris (IRD) in the midlatitude oceans and subpolar seas tells of widespread calving of icebergs from the Northern Hemisphere ice sheets during the last glacial period, but the climatic implications of this IRD are unclear. Does the sediment record indicate repeated dynamical collapse of the ice sheets, with ice sheets actively forcing the climate system? Alternatively, were ice sheet margins simply advancing and retreating in response to climate vacillations? On the basis of simulations of iceberg delivery to the ocean during the last glacial cycle we argue that the marine record exemplifies both of these phenomena. Heinrich events were clearly episodes of internal dynamical instability of the Laurentide Ice Sheet, while millennial-scale IRD is more simply interpreted as a response of the circum-Atlantic ice sheets to Dansgaard-Oeschger climate cycles. Ice sheets in different coastal regions respond differently to climate fluctuations, but overall iceberg fluxes increase in cold periods, peaking within a few centuries of climatic cooling. Regions with relatively warm, wet climates (Scandinavia, western North America, and Svalbard) are the most sensitive to millennial climate variability, with rapid response times and large millennial variability in iceberg fluxes.